Ambient Noise Imaging (ANI) of subsurface structures relies on seismic interferometry of diffuse seismic wavefields. However, the lack of effective methods to quantify and identify highly diffuse waves hampers applications of ANI, particularly in evaluating seismic attenuation and monitoring structural changes with high temporal resolution. Conventional ANI approaches require data normalization, which effectively suppresses the non-diffuse component with large amplitude but also results in significant loss of amplitude and phase information in the continuous seismic records. In this study, we propose a frequency domain method to quantitatively evaluate the degree of diffuseness of seismic wavefields by analyzing their statistical characteristics of modal amplitudes for stationarity and randomness. Tests on synthetic waveform and field nodal records show that the proposed method can effectively distinguish between diffuse and non-diffuse waveforms for either single- or three-component data. As an application, we identify a 60-s-long diffuse coda of a local M 2.2 earthquake recorded by a dense nodal array on the San Jacinto Fault Zone, and successfully extract high-quality dispersion curve and Q-value without performing data normalization. These results are consistent with those obtained by conventional methods that assess the correlation between coherency and the Green's function, and by modeling ballistic waves generated by road traffic. Our proposed method can advance the imaging of subsurface velocity and attenuation structures as well as monitoring temporal changes for scientific studies and engineering applications.

中文翻译：

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漫波场定量评估的频域方法及其在地震成像中的应用

地下结构的环境噪声成像 (ANI) 依赖于扩散地震波场的地震干涉测量。然而，缺乏有效的方法来量化和识别高度漫射波，阻碍了 ANI 的应用，特别是在评估地震衰减和监测高时间分辨率的结构变化方面。传统的ANI方法需要对数据进行归一化，这有效地抑制了大振幅的非漫射分量，但也导致了连续地震记录中振幅和相位信息的显着损失。在本研究中，我们提出了一种频域方法，通过分析地震波场的平稳性和随机性模态振幅的统计特征来定量评估地震波场的扩散程度。对合成波形和场节点记录的测试表明，该方法可以有效地区分单分量或三分量数据的漫射波形和非漫射波形。作为一个应用，我们识别了圣哈辛托断裂带上密集节点阵列记录的当地 M 2.2 地震的 60 秒长弥散尾波，并成功提取了高质量的频散曲线和Q值，而无需进行数据归一化。这些结果与通过评估相干性和格林函数之间的相关性的传统方法以及通过对道路交通产生的弹道波进行建模而获得的结果一致。我们提出的方法可以推进地下速度和衰减结构的成像，以及监测科学研究和工程应用的时间变化。